Catabolism Of Apo Research Articles

Abstract 527: Decay of Hepatic and Serum Apolipoprotein(a) Levels is Slower than ApolipoproteinB Following Inhibition of Lipid Synthesis

The association of Lipoprotein(a) (Lp(a)) levels and increased cardiovascular risk is substantiated by human epidemiology, genetics and interventional apheresis studies. Lp(a) is a plasma lipoprotein consisting of a Low Density Lipoprotein (LDL) particle with one molecule of apolipoprotein B100 (ApoB100) covalently linked to one molecule of apolipoprotein(a). Published studies show that apo(a) synthesis and secretion from hepatoma cells is coupled to triglyceride synthesis and secretion, illustrating a point of control for Lp(a) production. The objective of this study is to compare Lp(a), apo(a) and apoB production, secretion and half-life in vivo to understand mechanisms controlling circulating levels of Lp(a). Since rodent species do not express endogenous apo(a), human apo(a) transgenic mice were created and bred with human apoB100 transgenic mice to generate Lp(a)-producing mice. The circulating level of Lp(a) in the transgenic mice is ~3.7 mg/dL that is comparable to 5.4 mg/dL measured in a healthy human subject. To inhibit triglyceride and VLDL secretion in liver, mice were treated with a microsomal triglyceride transfer protein inhibitor (MTPi), and plasma was collected in a time-course. This treatment allowed measurement of plasma Lp(a), apo(a) and apoB half-lives since secretion of nascent VLDL and LDL was blocked. MTPi caused an 81% reduction in cholesterol and 68% reduction in triglycerides after 5 days of treatment. ApoB decreased significantly within 6 hours of treatment and remained low for 5 days. The calculated half-life was 4.6 hours. By contrast, apo(a) and Lp(a) decreased significantly after 3 days of treatment with half-lives of 2.4 and 2.5 days, respectively, illustrating delayed catabolism of apo(a)/Lp(a). Hepatic Lp(a), apo(a) and apoB were decreased significantly after 5 days dosing, indicating that the apolipoproteins were likely degraded and not accumulated in the liver. Treatment of stable HepG2 cells expressing apo(a) with MTPi caused similar difference in Lp(a) and apoB half-lives. These models illustrate rapid decay of apoB, but not apo(a), when lipoprotein particle assembly is blocked, and they provide methods for future mechanistic studies of apo(a) assembly into Lp(a) and its catabolism.

An Enzyme-Linked Immunosorbent Assay Method to Measure Human Apolipoprotein E Levels Using Commercially Available Reagents: Effect of Apolipoprotein E Polymorphism on Serum Apolipoprotein E Concentration

A sensitive and specific enzyme-linked immunosorbent assay (ELISA) for human apolipoprotein E (apo E) quantification using commercially available reagents is described. The assay is a noncompetitive, sandwich ELISA in which the wells were coated with a monoclonal EO1 antibody anti-human apo E and detected with a polyclonal antibody-peroxidase conjugate anti-apo E. The mean apo E concentration in 168 middle-aged subjects randomly selected from general population was 51.7 ± 12.4 mg/liter. Apo E levels were highly correlated with apo E phenotypes. Apo E polymorphism, which shows a modulating effect in the catabolism of apo E containing lipoproteins, may explain a large fraction, 18.5%, of the variability of serum apo E levels in middle-aged population. Isoforms apo E2 and apo E4 have an opposite effect on the regulation of serum apo E concentrations. Individuals that express apo E2 isoform present higher apo E levels (65.5 mg/liter for apo E2/E3), whereas the average of individuals with apo E4 is lower (42.8 mg/liter for apo E4/E3) than general population.